There has been proposed an apparatus for inputting a speech signal uttered in a manner inaudible to a third party.
In Non-Patent Document 1, for example, there is disclosed a NAM (Non-Audible Murmur) microphone, a device designed to receive a non-audible murmur. This NAM microphone is able to receive not only NAM but also an ordinary speech transmitted within a body, also termed ‘BTOS’ (body-transmitted ordinary speech).
In Patent Document 1, for example, there is disclosed a technique in which features of a spectral envelop of an unvoiced speech, input to an unvoiced speech input unit, are extracted, and the unvoiced speech is converted with the aid of the so extracted features (the unvoiced speech may be converted to an ordinary speech or an ordinary speech exhibiting the characteristic of another person). In Patent Document 2, there is disclosed a technique in which a NAM signal, received by a NAM microphone, is recognized as speech and a candidate for recognition is output by speech synthesis and checked. In Patent Document 3, there is disclosed a technique in which there are provided a first microphone for collecting an audible speech signal, a second microphone for collecting a non-audible speech signal (NAM) and a memory for holding signals received from the first and second microphones. The signal received from the second microphone is subjected to speech recognition, based on an acoustic model constructed by the non-audible sound, and the text, obtained by the speech recognition, is converted to a synthesized speech which is deemed to be an ordinary speech and recorded. In Patent Document 4, there is disclosed a configuration in which an interval where the audible sound exists and an interval where the non-audible sound exists are separately represented based on the signals delivered from the first and second microphones. In Patent Document 4, there is disclosed a technique in which the speech from a NAM microphone is recognized as speech and a microphone wearing location is guided to an optimum location based on e.g. a recognition model.